The enzymatic mechanism of glucuronidation catalyzed by two purified rat liver steroid UDP-glucuronosyltransferases

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

Kinetic analysis of Arabidopsis glucosyltransferase UGT74B1 illustrates a general mechanism by which enzymes can escape product inhibition

Plant genomes encode numerous small molecule glycosyltransferases which modulate the solubility, activity, immunogenicity and/or reactivity of hormones, xenobiotics and natural products. The products of these enzymes can accumulate to very high concentrations, yet somehow avoid inhibiting their own biosynthesis. Glucosyltransferase UGT74B1 (UDP-glycosyltransferase 74B1) catalyses the penultimate step in the core biosynthetic pathway of glucosinolates, a group of natural products with important functions in plant defence against pests and pathogens. We found that mutation of the highly conserved Ser284 to leucine [wei9-1 (weak ethylene insensitive)] caused only very mild morphological and metabolic phenotypes, in dramatic contrast with knockout mutants, indicating that steady state glucosinolate levels are actively regulated even in unchallenged plants. Analysis of the effects of the mutation via a structural modelling approach indicated that the affected serine interacts directly with UDP-glucose, but also predicted alterations in acceptor substrate affinity and the kcat value, sparking an interest in the kinetic behaviour of the wild-type enzyme. Initial velocity and inhibition studies revealed that UGT74B1 is not inhibited by its glycoside product. Together with the effects of the missense mutation, these findings are most consistent with a partial rapid equilibrium ordered mechanism. This model explains the lack of product inhibition observed both in vitro and in vivo, illustrating a general mechanism whereby enzymes can continue to function even at very high product/precursor ratios.

UDP-glucuronic acid binds first and the aglycone substrate binds second to form a ternary complex in UGT1A9-catalyzed reactions, in both the presence and absence of bovine serum albumin

The presence of bovine serum albumin (BSA) largely modulates the enzyme kinetics parameters of the human UDP-glucuronosyltransferase (UGT) 1A9, increasing both the apparent aglycone substrate affinity of the enzyme and its limiting reaction velocity (Drug Metab Dispos 39:2117-2129, 2011). For a better understanding of the BSA effects and an examination of whether its presence changes the catalytic mechanism, we have studied the enzyme kinetics of 4-methylumbelliferone glucuronidation by UGT1A9 in the presence and absence of 0.1% BSA, using bisubstrate enzyme kinetic experiments, in both the forward and reverse directions, as well as product and dead-end inhibition. The combined results strongly suggest that the reaction mechanism of UGT1A9, and presumably other human UGTs as well, involves the formation of a compulsory-order ternary-complex, with UDP-α-d-glucuronic acid (UDPGA) as the first binding substrate. Based on the enzyme kinetic parameters measured for the forward and reverse reactions, the equilibrium constant of the overall reaction was calculated (Keq = 574) and the relative magnitudes of the reaction rate constants were elucidated. The inclusion of BSA in the bisubstrate kinetic experiments quantitatively changed the apparent enzyme kinetic parameters, presumably by removing internal inhibitors that bind to the binary enzyme-UDPGA (E-UDPGA) complex, as well as to the ternary E-UDPGA-aglycone complex. Nevertheless, the underlying compulsory-order ternary-complex mechanism with UDPGA binding first is the same in both the absence and presence of BSA. The results offer a novel understanding of UGT enzyme kinetic mechanism and BSA effects.

Sterol glycosyltransferases--the enzymes that modify sterols

Sterols are important components of cell membranes, hormones, signalling molecules and defense-related biotic and abiotic chemicals. Sterol glycosyltransferases (SGTs) are enzymes involved in sterol modifications and play an important role in metabolic plasticity during adaptive responses. The enzymes are classified as a subset of family 1 glycosyltransferases due to the presence of a signature motif in their primary sequence. These enzymes follow a compulsory order sequential mechanism forming a ternary complex. The diverse applications of sterol glycosides, like cytotoxic and apoptotic activity, anticancer activity, medicinal values, anti-stress roles and anti-insect and antibacterial properties, draws attention towards their synthesis mechanisms. Many secondary metabolites are derived from sterol pathways, which are important in defense mechanisms against pathogens. SGTs in plants are involved in changed sensitivity to stress hormones and their agrochemical analogs and changed tolerance to biotic and abiotic stresses. SGTs that glycosylate steroidal hormones, such as brassinosteroids, function as growth and development regulators in plants. In terms of metabolic roles, it can be said that SGTs occupy important position in plant metabolism and may offer future tools for crop improvement.

Modulation of UDP-glucuronosyltransferase activity by protein-protein association

Drug oxidation and conjugation mediated by cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) have long been considered to take place separately. However, our recent studies have suggested that CYP3A4 specifically associates with UGT2B7 and alters the regioselectivity of morphine glucuronidation. This observation strongly supports the view that there is functional cooperation between P450 and UGT to facilitate multistep drug metabolism. In recent years, accumulating evidence has suggested an interaction between UGT isoforms or between P450 and UGTs and a change in UGT function by protein-protein association. In this review, we summarize these interactions and discuss their relevance to UGT function.

Purification and characterization of a novel glucosyltransferase specific to 27β-hydroxy steroidal lactones from Withania somnifera and its role in stress responses

Sterol glycosyltransferases catalyze the synthesis of diverse glycosterols in plants. Withania somnifera is a medically important plant, known for a variety of pharmacologically important withanolides and their glycosides. In this study, a novel 27beta-hydroxy glucosyltransferase was purified to near homogeneity from cytosolic fraction of W. somnifera leaves and studied for its biochemical and kinetic properties. The purified enzyme showed activity with UDP-glucose but not with UDP-galactose as sugar donor. It exhibited broad sterol specificity by glucosylating a variety of sterols/withanolides with beta-OH group at C-17, C-21 and C-27 positions. It transferred glucose to the alkanol at C-25 position of the lactone ring, provided an alpha-OH was present at C-17 in the sterol skeleton. A comparable enzyme has not been reported earlier from plants. The enzyme is distinct from the previously purified W. somnifera 3beta-hydroxy specific sterol glucosyltransferase and does not glucosylate the sterols at C-3 position; though it also follows an ordered sequential bisubstrate reaction mechanism, in which UDP-glucose and sterol are the first and second binding substrates. The enzyme activity with withanolides suggests its role in secondary metabolism in W. somnifera. Results on peptide mass fingerprinting showed its resemblance with glycuronosyltransferase like protein. The enzyme activity in the leaves of W. somnifera was enhanced following the application of salicylic acid. In contrast, it decreased rapidly on exposure of the plants to heat shock, suggesting functional role of the enzyme in biotic and abiotic stresses.

Purification and physico-kinetic characterization of 3beta-hydroxy specific sterol glucosyltransferase from Withania somnifera (L) and its stress response

Sterol glycosyltransferases catalyze the synthesis of diverse glycosteroids in plants, leading to a change in their participation in cellular metabolism. Withania somnifera is a medically important plant, known for a variety of pharmacologically important withanolides and their glycosides. In this study, a cytosolic sterol glucosyltransferase was purified 3406 fold to near homogeneity from W. somnifera leaves and studied for its biochemical and kinetic properties. The purified enzyme was active with UDP-glucose but not with UDP-galactose as sugar donor. It exhibited broad sterol specificity by glucosylating a variety of sterols and phytosterols with 3beta-OH group. It showed a low level of activity with flavonoids and isoflavonoids. The enzyme gave maximum K(cat)/K(m) value (0.957) for 24-methylenecholesterol that resembles aglycone structure of pharmacologically important sitoindosides VII and VIII from W. somnifera. The enzyme follows ordered sequential bisubstrate mechanism of reaction, in which UDP-glucose and sterol are the first and second binding substrates. This is the first detailed kinetic study on purified plant cytosolic sterol glucosyltransferases. Results on peptide mass fingerprinting and substrate specificity suggested that the enzyme belongs to the family of secondary metabolite glucosylating glucosyltransferases. The enzyme activity exhibited a rapid in vivo response to high temperature and salicylic acid treatment of plants, suggesting its physiological role in abiotic and biotic stress.

KINETIC CHARACTERIZATION OF THE 1A SUBFAMILY OF RECOMBINANT HUMAN UDP-GLUCURONOSYLTRANSFERASES

The initial glucuronidation rates were determined for eight recombinant human UDP-glucuronosyltransferases (UGTs) of the 1A subfamily, and the bisubstrate kinetics and inhibition patterns were analyzed. At low substrate concentrations, the reactions followed general ternary complex kinetics, whereas at higher concentrations of both substrates, the reactions were mostly characterized by ternary complex kinetics with substrate inhibition. The glucuronidation of entacapone by UGT1A9 was inhibited by 1-naphthol in a competitive fashion, with respect to entacapone, and an uncompetitive fashion, with respect to UDP-glucuronic acid (UDPGA). Its inhibition by UDP, on the other hand, was noncompetitive with respect to entacapone and competitive with respect to UDPGA. These inhibition patterns are compatible with a compulsory ordered bi bi mechanism in which UDPGA is the first-binding substrate. Despite the identical primary structure of the C-terminal halves of the UGT1A isoforms, there were marked differences in the respective K(m) values for UDPGA, ranging from 52 microM for UGT1A6 to 1256 microM for UGT1A8. Relative specificity constants were calculated for the eight UGT1A isoforms with 1-hydroxypyrene, 4-nitrophenol, scopoletin, 4-methylumbelliferone, and entacapone as aglycone substrates. The results demonstrated that seven of the UGT1A isoforms are capable of conjugating phenolic substrates with similar highest k(cat) values, and UGT1A4 has a lower relative turnover rate. The highest specificity constants were obtained for 1-hydroxypyrene, even with UGT1A6, which has been regarded as a specific isoform for small planar phenols. A k(cat) value of 1.9 s(-1) was calculated for the glucuronidation of scopoletin by purified UGT1A9.

UGT2B23, a novel uridine diphosphate-glucuronosyltransferase enzyme expressed in steroid target tissues that conjugates androgen and estrogen metabolites

Glucuronidation is widely accepted as a mechanism involved in the catabolism and elimination of steroid hormones from the body. However, relatively little is known about the enzymes involved, their specificity for the different steroids, and their site of expression and action. To characterize the pathway of steroid glucuronidation, a novel uridine diphosphate glucuronosyltransferase (UGT) enzyme was cloned and characterized. A 1768-bp complementary DNA, encoding UGT2B23 was isolated from a monkey liver library. Stable expression of UGT2B23 in human HK293 cells and Western blot analysis demonstrated the presence of a 51-kDa protein. The UGT2B23 transferase activity was tested with 62 potential endogenous substrates and was demonstrated to be active on 6 steroids and the bile acid, hyodeoxycholic acid. Kinetic analysis yielded apparent Michaelis constant (Km) values of 0.9, 13.5, 1.6, and 5.7 microM for the conjugation of androsterone (ADT), 3alpha-Diol, estriol, and 4-hydroxyestrone, respectively. RT-PCR analysis revealed that UGT2B23 transcript is expressed in several tissues, including the prostate, mammary gland, epididymis, testis, and ovary. Primary structure analysis shows that UGT2B23 is in the same family of enzymes as the previously characterized monkey isoforms UGT2B9 and UGT2B18, which are active on hydroxyandrogens. The characterization of UGT2B23 as a functional enzyme active on 3alpha-hydroxysteroids, and its expression in extrahepatic tissues, indicate that it may potentially play an important role in estrogen and androgen catabolism in peripheral steroid target tissues.

Structural and functional studies of UDP-glucuronosyltransferases

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Tricyclic antidepressants inhibit opioid receptor binding in human brain and hepatic morphine glucuronidation

The opioid receptor binding in the human thalamic area was studied with U-69593 and naloxone as ligands for the kappa and mu receptors, respectively. The binding was inhibited by various tricyclic antidepressants including amitriptyline, nortriptyline, clomipramine and fluoxetine. The antidepressants tested had a slight selectivity for the kappa receptor type. The IC50-values for all tricyclic antidepressants tested were in the 10(-6) M concentration range. Morphine and tricyclic antidepressants are substrates of a liver microsomal uridine diphosphate glucuronyl transferase (UDPGT). The interaction of the tricyclic antidepressants with morphine glucuronidation was investigated in human liver microsomal preparations. All drugs inhibited the morphine UDPGT. In Dixon plots inhibition of the formation of morphine-3-glucuronide and morphine-6-glucuronide was non-competitive for nortriptyline, and competitive or mixed for amitriptyline and clomipramine. Lubrol PX activated the morphine-UDPGT four to five times. The degree of activation of the enzyme(s) was unaltered in presence of the inhibiting drugs. The inhibition was also observed at a tricyclic antidepressant/morphine concentration ratio close to that achieved in plasma from patients treated with these drugs.

Cytosylglucuronic acid synthase (cytosine: UDP-glucuronosyltransferase) from Streptomyces griseochromogenes, the first prokaryotic UDP-glucuronosyltransferase

Cytosylglucuronic acid synthase (cytosine: UDP-glucuronosyltransferase), the first prokaryotic UDP-GT and a key enzyme in the biosynthesis of the antibiotic blasticidin S, was purified 870-fold. It has optimum activity at a pH of 8.4 to 8.6, Kms of 6.0 (UDP-glucuronic acid) and 243 (cytosine) microM, and a maximum rate of metabolism of 14.6 mumol/min/mg. The apparent M(r) is 43,000. Activity was slightly enhanced by Mg2+ or Ca2+ but was not inhibited by EDTA. Activity was strongly inhibited by UDP. Cytosylglucuronic acid differs from eukaryotic UDP-glucuronosyltransferases in being a soluble protein with no apparent phospholipid requirement.

Mechanistic studies of uridine diphosphate glucuronosyltransferase

Bisubstrate reaction kinetics and product inhibition studies were used to characterize the kinetic mechanism of a partially purified uridine diphosphate glucuronosyltransferase (UDPGT). These studies indicate that the reaction most likely occurs via a random order sequential mechanism. The effect of electron withdrawing and donating groups on the rate of reaction was also determined. It was found that electron donating groups increased the rate of glucuronide conjugation. This result is consistent with nucleophilic attack of the C-1 carbon of the UDP-glucuronic acid (UDPGA) by an SN2 mechanism. This is the first direct evidence for a SN2 mechanism in UDPGT catalysis.

Isolation and characterization of hyodeoxycholic-acid: UDP-glucuronosyltransferase from human liver

The enzyme hyodeoxycholic-acid: UDP-glucuronosyltransferase was purified about 230-fold from a solubilized human liver microsomal preparation utilizing anion-exchange chromatography, ampholyte-displacement chromatography and UDP-hexanolamine--Sepharose affinity chromatography. The homogeneity of the final enzyme preparation was judged by two criteria: the appearance of a single band of Mr 52000 in SDS/PAGE; the elution of a single peak in reversed-phase FPLC. The isolated enzyme catalyzed the glucuronidation of the 6 alpha-hydroxy bile acids hyodeoxycholic and hyocholic acids, and of the steroid hormone estriol, with a ratio of relative reaction rates of 13:1:2.7. UDP-glucuronosyltransferase activities toward the 3 alpha-hydroxy bile acid lithocholic acid, androsterone, testosterone, bilirubin and p-nitrophenol were not detectable in the pure enzyme preparation and were shown to be separated from enzyme activity toward hyodeoxycholic acid during ampholyte-displacement chromatography and/or UDP-hexanolamine--Sepharose affinity chromatography. Two-substrate kinetic analysis of hyodeoxycholic-acid-conjugating activity gave a sequential mechanism with apparent Km values of 12 microM and 4 microM for hyodeoxycholic acid and UDP-glucuronic acid, respectively. Phospholipids were required for reconstitution of maximal activity toward hyodeoxycholic acid. Phosphatidylcholine was the most effective activator of enzyme activity.

Endogenous substrates for UDP-glucuronosyltransferases

1. Multiple forms of UDP-glucuronosyltransferase (UDPGTs) have been demonstrated in the livers of all mammalian species that have been studied. Rat liver possesses at least eight different isozymes and human liver has at least five different forms which have been identified. 2. Endogenous substrates (e.g., steroids) are helpful in distinguishing UDPGTs as they generally react with only a single form, whereas xenobiotic substrates (e.g., 4-methyl-umbelliferone, p-nitrophenol) react with several forms of the enzyme. 3. Human liver UDPGTs differ in physical properties and substrate specificity from these enzymes obtained from laboratory animals. Hence, it is necessary to study human liver UDPGTs to elucidate substrate specificity and to understand drug-endogenous substrate interaction in humans.